Power supplies, in order to be most efficient, must provide a load to the primary power source such that the power factor of the voltage and current into the power supply is close to 1.0. Any degradation of this power factor results in power which is not available to the power supply, and thereby requires the primary power source to be able to provide more apparent power than the actual power necessary. As a result power factor input specifications, such as IEEE 519, are more and more being placed on power supplies.
The power factor, the real power divided by the apparent power, is determined principally by two factors: The phase relationship between the voltage and current, and the distortion of the current.